James- Is her face still numb, or did it subside?
Sudden numbness in the face could be something called a TIA (transient ischemic attack).The official definition of a TIA is the sudden onset of neurological signs of a "brain attack" (i.e. numbness or weakness of face, arm or leg) that completely resolve in 24 hours. In practice, TIAs usually last 5-10 minutes.
A TIA is caused by a temporary state of reduced blood flow (called "ischemia") in a portion of the brain. This is most frequently caused by tiny blood clots that temporarily block blood flow to a portion of the brain.
I don't know if this is what happened with her but she should see a doctor because it could be a serious warning sign of a future stroke.
Definitely not to be ignored, even if it has long gone. She should have a thorough physical and certainly have her blood pressure evaluated.
It could have been a TIA, as I said, and the operative word is "transient", which hers was...Please don't ignore this. If there is a problem, she's in a very good place to do something about it and head of what may be a catastrophic event (stroke) later.